The present invention relates to a calcination apparatus for use in a fluidized-bed burning installation for powdery raw materials and more particularly a calcination apparatus used in an installation in which powdery raw materials for cement or alumina are burned in a fluidized-bed burning furnace.
A conventional fluidized-bed burning installation for cement has a construction as shown in FIG. 3 in which reference numeral 1 designates means for supplying cement powdery raw materials; 2, a main exhaust fan; 3, a first cyclone of a suspension preheater; 4, a second cyclone thereof; 5, a third cyclone thereof; 6, a fourth or lowermost cyclone thereof; 7, a calcining furnace; 8, a burner; 9, a vortex chamber which is disposed immediately below the calcining furnace 7 and supplies the swirling secondary burning air to the furnace 7; 10, a burner; 11, a fluidized-bed burning furnace; 12, a burner; 13, a riser duct for supplying high-temperature exhaust gas (1300.degree.-l350.degree. C.) from the fluidized-bed burning furnace 11 into the vortex chamber 9; 14, a chute for supplying the preheated raw materials to the calcining furnace 7 and riser duct 13; 15, a horizontal duct for connecting the calcining furnace 7 with the lowermost cyclone 6; 16, a chute for supplying the calcined powdery raw materials from the lowermost cyclone 6 to the fluidized-bed burning furnace 11; 17, a classifier for classifying cement clinker burned in the fluidized-bed burning furnace 11; 18, a primary cooler for air cooling the high-temperature cement clinker; 19, a secondary cooler; and 20, a duct for supplying the secondary combustion air required in the calcining furnace 7 from the secondary cooler 19 to the vortex chamber 9. The solid-line arrows indicate the flows of the air and gas while the broken-line arrows indicate the flows of the powdery raw materials and clinker.
The powdery raw materials are preheated by exhaust gas as they flow through the cyclones 3, 4 and 5 and ducts therebetween. The preheated powdery raw materials from the cyclone 5 pass through the chute 14 into the calcining furnace 7 and riser duct 13. Thereafter they are calcined in the calcining furnace 7 and flow through the horizontal duct 15 into the lowermost cyclone 6 and then are charged through the chute 16 into the fluidized-bed burning furnace 11 where they are burned into cement clinker.
When hydraulic modulus of cement raw materials expressed by ##EQU1## increases in excess of 2.2, growth of clinker particles in the fluidized-bed burning furnace 11 is stopped unless temperature in the furnace 11 is increased as high as 1350.degree. C. Clinker particles whose growth is stopped scatter and are entrained in the flowing exhaust gas to circulate through the fluidized-bed burning furnace 11, the riser duct 13, the vortex chamber 9, the calcining furnace 7, the horizontal duct 15, the lowermost cylcone 6, the chute 16 and again the fluidized-bed burning furnace 11. As a result, the low-temperature clinker particles are returned to the fluidized-bed burning furnace 11 and the temperature in the fluidized-bed burning furnace 11 cannot be raised as high 1350.degree. C. and consequently there arises a problem that the cement production capacity is adversely affected.
When the clinker particles less than two millimeters in particle size are entrained in the flowing exhaust gas from the fluidized-bed burning furnace 11, pass through a freeboard in the fluidized-bed burning furnace 11 and then flow into the riser duct 13 through which the gas flows at a high velocity (about 25 meters per second), they cannot drop into the fluidized-bed burning furnace 11, but flow upward into the calcining furnace 7. They cannot however flow through the inner low-velocity portion (about four meters per second) in the calcining furnace 7 and they stay in the calcining furnace 7, forming a spouted bed. As a result, draft variations occur so that there arises a problem that balance of the system is worsened because the draft variations adversely affect dust collection efficiency (that is, efficiency for separating the powdery raw materials from the exhaust gas) of the cylcones 3, 4 and 5 of the upstream suspension preheater.
Futhermore, in order to maintain calcination degree at about 85% without clogging trouble, the gas temperature at the exit of the lowermost cyclone 6 must be maintained at a set value of 860.degree. C. However, in the conventional calcining furnace 7, the spouted bed suppresses the whirling effect of the secondary combustion air charged into the vortex chamber 9 so that the powdery raw materials charged into the calcining furnace 7 are not sufficiently swirled and the short pass results, causing decrease of retension time of the raw materials in the calcining furnace 7, and also the combustion efficiency of the burners 8 and 10 is adversely affected. As a consequence, the calcination efficiency cannot be kept properly. In order to attain the calcination degree of 85%, the gas temperature at the exit of the lowermost cyclone 6 must be maintained at about 870.degree. C. Then the gas temperature at the exit of the third cyclone 5 increases as high as about 790.degree. C. so that there arises a problem that the powdery raw materials become viscous, causing clogging of the third cyclone 5.
The present invention was made to overcome the above and other problems encountered in the prior art and has for its object to provide a calcination apparatus for use in a fluidized-bed burning installation for powdery materials in which calcination efficiency is improved in a calcining furnace; dust collection efficiency of the cylcones in the suspension preheater is improved, thus attaining energy savings; and pulsation in the calcining furnace is minimized to maintain a high degree of balance of the whole system.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of a preferred embodiment thereof taken in conjunction with the accompanying drawings.